More than half of all American troops based in Afghanistan will be withdrawn from duty over the next 12 months.

Barack Obama is set to make the announcement in his State of the Union address on Tuesday night, a source familiar with his speech has confirmed.

There are currently 66,000 US service members in Afghanistan.

The final withdrawal of most foreign combat troops is not due until the end of 2014.

There were no immediate details on how quickly the drawdown would take place.

Military experts suggest it is unlikely to happen until late Autumn so as not to adversely impact the number of troops NATO will have in place to fight the Taliban after the spring thaw in Afghanistan.

A residual force to train Afghan soldiers and to conduct anti-terror missions will remain behind after the formal withdrawal.

Barack Obama prepares for his State of the Union address

Mr Obama has made ending the wars in Iraq and Afghanistan the centrepiece of his presidency, and often declares a long decade or more of American war is almost over.

The president is also likely to use Tuesday's address to Congress to expand upon his other goals for the year.

They include job creation and a push for the ambitious progressive plans he outlined in his second inaugural address three weeks ago.

Mr Obama hopes to encourage lawmakers to join him in reforming laws on gun ownership and immigration and boosting taxes to raise government spending power.

He will also address North Korea's announcement that it successfully detonated a nuclear device earlier on Tuesday in defiance of UN warnings.

The White House said the president would make the case that the nuclear program had only further isolated the impoverished nation.